The present invention relates to phase coded, pulse compression radars, and more particularly, to a pulse compression signal processor suitable for use therein for decoding the phase-coded radar return pulses to enhance the resolvability of objects in range.
Phase-coding and pulse compression are commonly used in radar systems to effect an increased average power in pulse transmission while maintaining a fine range resolving capability in processing the phase-coded radar return pulses. These techniques may be accomplished by transmitting a long phase-coded or phase-modulated pulse and by signal processing a return pulse, rendered from the transmission, to a relatively short pulse, commonly referred to as pulse compression.
The block diagram schematic embodiment depicted in FIG. 1 exemplifies a phase-coded, pulse compression radar. Typically, a stable local oscillator (STALO) 10 generates an RF signal 12 which may be gated at 14 using a signal 16 representative of a predetermined pulse width to form a substantially pulsed RF signal 17. A phase modulator 18 may be used to code the RF of the pulse in accordance with a phase code signal 20 which may be generated by a phase code generator 22. The phase-coded pulsed RF signal governs the operation of a conventional transmitter 24 for transmission of the pulse into space via circulator 26 and antenna system 28. A return radar pulse is received by the antenna system 28 and conducted to a conventional receiving section 30 via circulator 26. The receiver section 30 conditions the phase-coded RF return pulse conventionally using a plurality of mixer stages 32, which may be governed by various local oscillator signals LO and corresponding amplifier and low-pass filter (LPF) stages 34.
In most modern radars, processing of the conditioned radar return signal is performed in a digital signal processor depicted at 36, for example, in which case the return phase-coded pulse signal is digitized first in an analog-to-digital (A/D) converter 38. Pulse compression of the radar return pulse may be accomplished by a phase decoding function 40 programmed into the digital signal processor 36. The phase-decoding function 40 utilizes a phase-coded signal 42 provided to the processor 36 from the phase-coded generator 22, for example, to pulse compress the digitized conditioned return pulse denoted by the arrow 44 to effect a signal 46 representative of the range correlation response thereof.
Not only does the phase decoding function 40 provide a response at the range of the object, it also generates, at times, responses of troublesome levels at ranges on either side thereof, more commonly referred to as range sidelobes. It has been generally an object in the design of phase decoding filters of this type to suppress the range sidelobes in the range correlation response. Some recent advances towards meeting this objective include a phase decoding filter design using least square methods which provide optimal performance in terms of the integrated sidelobe level. The least square method type filters have also been found to have very low signal-to-noise (S/N) loss. For a better understanding of least square method filters, reference is made to the paper: M. H. Ackroyd and F. Ghani, "Optimum Mismatched Filters for Sidelobe Suppression," IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-9, No. 2, Mar. 1973. One shortcoming of the least square method type filter is that the range correlation response usually contains a number of high peak range sidelobes since the peak sidelobe level is not explicitly minimized by the least square solution. As a result, these resulting high peak range sidelobes may give rise to false signals at ranges other than the range of the object of interest.
Another technique for suppressing the range sidelobes in the phase-coded pulse return signals, known as linear programming, was used to compute the weights of a phase decoding filter for minimizing the peak range sidelobes of the range correlation response. For a more detailed description of a linear programming technique, reference is made to the paper: S. Zoraster, "Minimum Peak Range Sidelobe Filters for Binary Phase Coded Waveforms," IEEE transactions on Aerospace and Electronic Systems, Vol. AES-16, No. 1, January 1980. This technique, however, is limited to binary phase codes only. In addition, the mathematical structure of the linear programming technique is such that it has at least as many inequality constraints which are exactly satisfied by the optimal solution as there are variables in the problem. This drawback results in increased integrated sidelobe level and S/N loss.
Apparently, what is desirable in pulse compression processing is to have a phase decoding filter which provides not only low integrated sidelobe level and low S/N loss, but also minimizes the peak sidelobe level. It would be therefore, beneficial to have a filter which did not suffer from a rigid mathematical structure in its optimization procedure, but rather permit a desired flexibility in the effectuation of the range correlation response of a phase-coded pulse return signal. Still further, the phase decoding filter should not be limited to solely binary phase-coded RF pulsed waveforms, but rather be additionally applicable to the polyphase codes of the more sophisticated modern radars.